The present invention relates to an improved single sideband transceiver ideally suited for citizen's band operation.
Single sideband operation is desirable in citizen's band communication from the standpoint of reductions in operating power, bandwidth and interference. In citizen's band class "D" operation, the maximum allowable frequency deviation is .+-.0.005%, or about .+-.1350 Hz. While a frequency deviation near the maximum limit causes no major problem in typical AM transmission, it renders the transmission unintelligible in single sideband (SSB) transmission. Generally, in the SSB mode the frequency may deviate by only about 50 Hz to 100 Hz if articulation is to be tolerable.
Unless a SSB transceiver is provided with a highly stable crystal oscillator maintained in a constant temperature enclosure, the frequency deviation will generally be as high as .+-.1000 Hz. However, the frequency deviation must be .+-.25 Hz to .+-.50 Hz for proper articulation, which is 1/27 to 1/54 of the statutory maximum permissible deviation. To ensure intelligibility, the frequency deviation must be 1/50 that of an oscillator of the type normally used in AM transmission. Oscillators of such stability are quite expensive and may make the cost of a SSB citizen's band transceiver commercially unattractive.
The problem may be overcome by providing an inexpensive oscillator in a transceiver in combination with a manual local oscillator frequency adjustment. After selecting the desired channel, the operator rotates the adjustment until he achieves maximum articulation or clarity. Such an adjustment is generally referred to as a fine tuning or clarifier control. However, such a system is a safety hazard in transceivers employed in automobiles since it distracts the driver from the operation of the vehicle. Safe drivers will generally switch the transceiver to the AM mode, if available, in order to eliminate the need for fine tuning. This, of course, results in a reduction of power and an increase in interference.